The present invention relates generally to base stations in code division multiple access (CDMA) wireless systems and other types of wireless communication systems, and more particularly to base stations capable of supporting multiple communication standards within such systems.
The rapid pace of development in wireless communication systems has typically led to significant changes to the standards which define the operation of such systems. For example, the standards defining the operation of IS-95 CDMA wireless systems have progressed from TIA/EIA IS-95A to TIA/EIA IS-95B, and are now in the process of moving toward TIA/EIA IS-2000, also known as IS-95C. The IS-95A, IS-95B and IS-95C standards are collectively referred to herein as IS-95. Other CDMA standards, such as Multi-Carrier (MC) cdma2000 and the next-generation European standard known as Universal Mobile Telecommunication System (UMTS), are also being proposed.
These related standards each generally define an air interface specification that allows a mobile unit to communicate with a base station associated with a cell site. The interface definition typically includes a set of air interface channels, channel signal encoding rules, and signaling messages to enable the mobile unit to place and receive voice or data calls to and from a land line network, as well as to and from other mobile users. However, when the differences between successive generations of standards are significant, the base stations designed to support one standard often cannot easily be changed to support the next generation of the standard, thereby necessitating a new base station design. In many cases, this need for a new base station design arises because new air interface specifications require circuit packs in the base station to communicate different sets of signals than those communicated in accordance with a previous version of the standard. This situation will be illustrated in conjunction with FIGS. 1 and 2 below.
FIG. 1 shows an example of a base station 100 configured in accordance with the above-noted. IS-95 standard. The base station 100 includes a control computer 102, a control and traffic bus 104, and a set of M channel unit boards 106-i, i=1, 2, . . . M. The control computer 102 interfaces with a mobile switching center (MSC) which provides a link to other base stations and to a public switched telephone network (PSTN). In an IS-95 CDMA system, spread spectrum digital signals from different user calls on a given base station antenna sector are added together to generate a composite spread spectrum digital signal for that sector. Individual spread spectrum digital signals are generated by channel elements, such as cell site modems (CSMs), that are part of the channel unit boards 106, and are combined to form the composite spread spectrum digital signal for a given sector. The base station design of FIG. 1 allows the channel unit boards 106 to communicate signals from one such board to the next in support of users on one CDMA carrier, designated C1, and up to three 120xc2x0 antenna sectors, designated xcex1,xcex2 and xcex3. Three sector systems are commonly used in practice, although omni-directional and two-sector systems may also be deployed. The use of a larger number of sectors, such as six sectors, is less common, but also possible.
Within each channel unit board 106-i in the base station 100 of FIG. 1, the spread spectrum digital signals of up to N users are added together on a per-sector basis. For each sector, the summed spread spectrum digital signals of users served by a particular channel unit board 106-i are added to the respective signals from the previous channel unit board, i.e., the channel unit board to its left in the FIG. 1 design. The summed digital signals are output from the channel unit board 106-i, and become inputs to the next-in-line channel unit board 106-(i+1) closer to a set of three radio boards 108-1, 108-2 and 108-3 in FIG. 1. Therefore, up to N users per channel unit board are added together by the mechanism of summing the signals from channel unit board to channel unit board. In a design with M such channel unit boards, each supporting up to N users, up to Mxc3x97N total users can be supported on the three sectors xcex1,xcex2 and xcex3. The interconnections between the channel unit boards are provided by a transmit digital""signal communications bus denoted Tx-bus.
It should be noted that although the description herein will be directed primarily to the transmit operations of the base stations, similar interconnection issues arise with respect to receive operations. The corresponding receive bus (Rx-bus) is omitted from FIG. 1 and other similar base station illustrations herein for purposes of clarity.
The digital processing elements on each of the channel unit boards 106-i can be used to support a user call on any of the three sectors xcex1, xcex2 and xcex3. This capability is referred to as channel element pooling, and in the FIG. 1 design, is applied to one carrier and three sectors. Digital in-phase (I) and quadrature phase (Q) signals, for each of the three sectors xcex1,xcex2 and xcex3 and the one CDMA carrier C1 are summed from channel unit board to channel unit board, and finally are passed to one of the three radio boards 108-1, 108-2 and 108-3, depending on the sector. Each radio board 108-1, 108-2 and 1,08-3 converts the digital I and Q signal inputs into an RF signal. The RF signals for sectors xcex1, xcex2 and xcex3 are then amplified by power amplifiers 110- 1, 110-2 and 110-3, filtered in transmit filters 121-1, 112-2 and 112-3, and radiated by transmit antennas 114-1, 114-2 and 114-3, respectively. Other types of conventional techniques may be used to communicate signals among the channel unit boards, e.g., the I and Q signals for each sect or may be multiplexed onto one back plane trace.
A basic problem with conventional base station designs such as that shown in FIG. 1 is the configuration of the transmit digital signal communications bus (Tx-bus) that interconnects the channel units boards 106. More particularly, it is generally very difficult to be able to redefine the bus according to the particular version of the standard that is being implemented, and according to the set or subset of features that are to be provided in a specific configuration of a given base station. The Tx-bus also needs to be able to support the radio boards used in the base station, and these radio boards likewise need to be capable of interpreting the, communications bus signals in different ways, depending on configuration commands they receive from a control program. Although the digital processing boards and the radio boards may be hard wired for specific bus signal usage,several board design types would then be required to cover all versions of the standards.
FIG. 2 illustrates the manner in, which the FIG. 1 base station design can be extended to support an additional CDMA carrier C2. Since the IS-95A RF signal occupies a bandwidth of 1.25 MHz, it is possible and desirable for base stations to support multiple CDMA carriers. However, the base station design of FIG. 1 generally cannot simply incorporate additional channel unit boards 106 in a direct way to provide service on the second CDMA carrier C2. Instead, the board interconnect structure of FIG. 1 needs to be completely replicated, in the manner show in FIG. 2, in order to provides service on the second carrier C2. The FIG. 2 base station 100xe2x80x2 therefore includes an additional set Of channel unit boards 116-1, . . . 116-M. The base station 100xe2x80x2 also includes an additional set of radio boards 118-1, 118-2 and 118-3, power amplifiers 120-1, 120-2 and 120-3, and filters 122-1, 122-2 and 122-3, for processing signals associated with sectors xcex1, xcex2 and xcex3, respectively, and carrier C2. In the base station 100xe2x80x2, channel element pooling is restricted to each carrier C1 or C2, but across the three sectors xcex1, xcex2 and xcex3 on each carrier. In other words, channel element pooling does not extend across carriers. The base station 100xe2x80x2 includes a pair of Tx-buses, each corresponding generally to the Tx-bus of FIG. 1.
The FIG. 1 base station design can be further extended in a similar manner to support more than two CDMA carriers. However, FIG. 2 illustrates that with respect to channel element pooling, the FIG. 1 design becomes non-extensible across carriers, such that a new base station design would be required in order to achieve a cross-carrier channel element pooling capability.
An example of the manner in which a change in standard can require a new base station design will now be provided with reference to FIGS. 3 and 4. The above-noted IS-95C standard in its current form incorporates new capabilities not found in its predecessor standards IS-95A and IS-95B, including a capability called Orthogonal Transmit Diversity (OTD) which offers additional system capacity under certain low mobility situations. With OTD, two sets of digital I and Q signals for each sector are created, are used to modulate a carrier frequency, and are radiated from different antennas.
FIG. 3 shows schematically the processing required in implementing OTD for a single call. A bit stream of a corresponding user signal on sector xcex1 is processed to form I1 and Q1 bit streams destined for one antenna of sector xcex1, and I2 and Q2 bit streams destined for another antenna of sector xcex1. A given bit in each group of four bits from the input bit stream is assigned to one of the bit streams I1, QI, I2 and Q2. These streams are then multiplied in multipliers 151, 152, 153 and 154, respectively, by either a Walsh_a or Walsh_b 256-bit spreading code to generate corresponding spread signals which are processed through digital processing boards 160-1 and 160-2, radio boards 162-1 and 162-2 providing modulation with a carrier C, power amplifiers 164-1 and 164-2, and transmit filters 166-1 and 166-2, and then transmitted via sector xcex1 antennas 168-1 and 168-2.
FIG. 4 shows a base station 200 designed in a conventional manner to support the above-described OTD capability of IS-95C. The base station 200 includes control computer 102, control and traffic bus 104, and a set of M channel unit boards 206-i, i=1, 2, . . . M. Each of the M channel unit boards supports N users, and provides I and Q signals for each of first and second antennas of the sectors xcex1, xcex2 and xcex3, i.e., for xcex11 and xcex12, for xcex21 and xcex22, and for xcex31 and xcex32. The base station 200 also includes sets of C1 radio boards 208, power amplifiers 210, transmit filters 212 and antennas 214, arranged as shown. Each of these sets includes a particular one of the C1 radio boards 208, power amplifiers 210, filters 212 and antennas 214, and supports a corresponding set of I and Q signals associated with a particular first or-second antenna of the sectors xcex1, xcex2 and xcex3. The digital Tx-bus in this example includes a separate signal line for each of the I and Q signals associated with each of the first and second antennas of the antenna sectors xcex1xcex2 and xcex3.
By comparing the FIG. 4 base station design with the FIG. 1 design, it can be seen that the FIG. 1 design generally cannot be used to support OTD unless it is possible to multiplex two sets of I and Q signals on the back plane traces of the channel unit boards. While it may be possible to perform this type of multiplexing, the signal rates are likely to be high enough to make for an unstable design. In any event, such a design still does not allow channel element pooling across multiple CDMA carriers; Separate instances of the sets of elements 206, 208, 210 and 212 of the FIG. 4 design would therefore need to be completely replicated in a single base station to implement this type of pooling across multiple CDMA carriers.
Another example of the manner in which a change in standard can require a new base station design will now be described with reference to FIG. 5. A development effort is underway for a wide band CDMA system which includes a downlink, i.e., base-to-mobile, signal that is constructed from three contiguous IS-95 carriers. This is the so-called Multi-Carrier (MC) cdma2000 approach, which specifies a CDMA signal occupying approximately 5 MHz of spectrum. The cdma2000 downlink signal is shown in FIG. 5. Instead of the user signal being spread directly across the bandwidth occupied by three IS-95 contiguous carriers C1, C2 and C3, the signal is split into three appropriate parts, where each part is processed separately, converted to an IS-95-like spread spectrum digital signal, and then used to modulate one of the three IS-95 carriers C1, C2 and C3, which are transmitted simultaneously.
An advantage of the above-described MC cdma2000 approach is that wireless system operators with systems configured in accordance with IS-95A or IS-95B can provide a wide band service with relatively modest investment in new equipment, i.e., the RF components used for IS-95A or IS-95B service can be used simultaneously to provide the wide band service. However, despite the fact that the RF components can be reused, the interconnection of the digital components shown in FIG. 1 generally cannot be used to support the MC cdma2000 standards specification. A new base station design for the digital processing components would therefore be necessary, because the signals, from three CDMA IS-95 carriers on three sectors would have to be available to the channel unit boards. The MC cdma2000 channel unit boards differ from those used in IS-95 because of the different signal processing required.
Instead of using multiple IS-95 CDMA carriers to construct the above-described cdma2000 signal, the user signal could be directly spread with a code signal at three times the spreading rate used in IS-95A or IS-95B. A so-called Direct-Spread: (DS) signal is then created. However, the design of FIG. 1 cannot be used to implement a DS approach, unless it is possible to send the I and Q signals at three times the rate used for IS-95A or IS-95B. This is not likely for current base station designs. Consequently, a new base station design would therefore generally be required to implement the DS approach.
In order to avoid expensive and lengthy development processes, and to provide investment protection to purchasers of base station hardware, it is highly desirable that a base station design be easily upgradable to support subsequent versions of a communication standard. However, as illustrated above for the case of IS-95 CDMA, it has generally proved difficult to design base station equipment that is readily extensible when new capabilities are added to the standard. A need therefore exists for an improved base station design which overcomes the extensibility issues described above.
The present invention provides a reconfigurable base station which is designed to be readily extensible to accommodate changes in operating standards. Unlike the prior art base station designs described above, which generally utilize a separate communications bus for each CDMA carrier, a base station in accordance with the present invention includes a reconfigurable communications bus with signal paths that may be configured to accommodate many different combinations of signals associated with a particular arrangement of carriers, sectors and antennas. Particular user signals can be assigned to designated signal lines of the reconfigurable bus in order to support a particular wireless system.
In accordance with the invention, a reconfigurable base station suitable for supporting multiple wireless communication system standards includes a set of channel unit boards, each providing processing operations for user signals assigned to multiple carriers of the system, a set of radio boards, each generating an RF output signal for each of at least a subset of the multiple carriers, and a reconfigurable bus interconnecting the channel unit boards and radio boards. The base station is configured to support a particular wireless system standard, such as IS-95 CDMA, by assigning particular user signals to designated signal lines of the reconfigurable bus.
The base station may then be reconfigured to support other CDMA standards, such as, e.g., IS-95C with or without Orthogonal Transmit Diversity (OTD), Multi-Carrier (MC) cdma2000 or Universal Mobile Telecommunications System (UMTS), by assigning other user signals to the signal lines of,the reconfigurable bus. The assignment of signal lines may be implemented dynamically under the control of configuration commands generated by software running on abase station control computer and supplied to the channel unit boards and radio boards. As another example, the assignments may be implemented by establishing fixed connections between the bus signal lines and appropriate ports of the channel unit and radio boards.
Advantageously, the invention allows base station digital processing resources to be pooled across all the CDMA carriers in a given configuration. This pooling is generally not possible in the above-described conventional base station designs. In addition, the invention protects the investments of base station equipment purchasers, by allowing existing equipment to be easily and efficiently upgraded to support changes in operating standards. Furthermore, the invention allows the cost-effective and space-effective deployment of equipment meeting new standards, while also simultaneously providing support for older standards. These and other features and advantages of the present invention will become more apparent from the accompanying drawings and the following detailed description.